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Focal Extender....any point?


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With galaxy season here, I would like to challenge myself to get some of the more elusive little blighters. I image with an ASI533MC Pro and I could use the RC6's long focal length (1350mm), but it's very slow at f9. I also have a Vixen R200SS (800mm f4) and Vixen do a focal extender specifically for that which would take it to 1120mm and still a reasonably fast f5.6. To me, that seems more sensible of the two.

My question is, would that give me any benefit over using the scope at its native 800mm focal length and simply cropping seeing as it would result in a resolution of 0.69"/pixel?

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9 minutes ago, osbourne one-nil said:

My question is, would that give me any benefit over using the scope at its native 800mm focal length and simply cropping seeing as it would result in a resolution of 0.69"/pixel?

Since you are using OSC camera - yes it would if you process OSC data properly and without interpolation.

0.69"/px is far to high sampling rate, but OSC cameras are in effect sampling at half of that - so 1.4"/px - that is very good working resolution for 8" scope (depends on the mount and your guiding performance as well).

Use extender, and do super pixel mode or equivalent instead of interpolation.

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If I''m doing interpolation, I don't know I'm doing it! When I image I dither every 3rd shot and when I process I tend to stack using the default settings in DSS, then stretch in Pixinsight, photoshop, twiddle. 

I see what you're saying about the sampling as I guess it's to do with the pixels being split between the bayer matrix? That's very a very useful point!

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11 minutes ago, osbourne one-nil said:

If I''m doing interpolation, I don't know I'm doing it! When I image I dither every 3rd shot and when I process I tend to stack using the default settings in DSS, then stretch in Pixinsight, photoshop, twiddle. 

I see what you're saying about the sampling as I guess it's to do with the pixels being split between the bayer matrix? That's very a very useful point!

If you are using DSS - default settings are using interpolation to debayer and recover color information.

This process allows to recover "full resolution" - so 3000x3000 color image, although there are in reality only 1500x1500 red pixels in bayer matrix (blue as well, and x2 this green ones).

Here is how to setup DSS to use super pixel mode - it will result in 1500x1500 px image though:

image.png.36fd186fc290baa0d5b36d4bd19065cc.png

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That’s incredibly helpful of you. Thank you! 
 

I guess the 1500x1500px image contains the same level of information than the larger image I currently get? 
 

I shot the Rosette Nebula the other night and was thrilled with the result. I’ll try processing it again and see what I get. 
 

Thanks again!

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I've had a little fiddle and here are the same shots. All I've done is applied a screen transform function and convert to jpeg. I've not done anything else (as may well be apparent!)

The first one was stacked the normal way in DSS and the 2nd using the super pixel technique. I'm not sure I can see a difference but the file's certainly smaller!

 

Debayered.jpg

Superpixel.jpg

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1 hour ago, osbourne one-nil said:

I guess the 1500x1500px image contains the same level of information than the larger image I currently get? 

Yes, it contains the same level of detail - if you process two images the same, you should get the same result.

Only difference is that 3000x3000px one is 200% "zoomed in" versus 1500x1500 one.

image.png.c96e5b95ec03c9b37f41f0121d21f2ce.png

This is however too much of a zoom - everything there is in the image can be seen in 1500px version - although it is smaller - it is also sharper:

image.png.d50f7f56ba44c2868d64e1722db54a4a.png

 

 

 

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On 12/02/2022 at 15:05, osbourne one-nil said:

With galaxy season here, I would like to challenge myself to get some of the more elusive little blighters. I image with an ASI533MC Pro and I could use the RC6's long focal length (1350mm), but it's very slow at f9. I also have a Vixen R200SS (800mm f4) and Vixen do a focal extender specifically for that which would take it to 1120mm and still a reasonably fast f5.6. To me, that seems more sensible of the two.

My question is, would that give me any benefit over using the scope at its native 800mm focal length and simply cropping seeing as it would result in a resolution of 0.69"/pixel?

Great to hear from another R200SS owner! There aren't many of us out there it seems. Although i don't know why. Japanese optics, lightweight, and with the right accessories (Starizona Nexus, Vixen Coma Corrector, and Vixen Extender) it can effectively triple up as 3 different scopes. I picked up an old one (maybe from the mid noughties) last year for a very good price, with the idea of using it initially as a Galaxy scope. It's nowhere near ready for first light (probably won't be until next year tbh) but mine came with the old Vixen Extender which is F7.5 and 1,500mm, rather than the newer (and much better) F5.6 1,120mm one. I'm going to give it a go anyway and see what it can do. If it's no good, then i'll eventually add the newer Extender. My camera is a Qhy268m, so i'll be even more oversampled than yourself (about 0.52"), so i'll likely just downsample in post. 

Will be interested to see how you get on with your R200SS. Good luck! 

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Nice! I don't quite get Vixen's relatively low-key profile. Perhaps they don't cost enough? Make them Takahashi prices and you're on to a winner!

I think my scope is about 8 years old but I like it so much I am almost tempted to buy a brand new one for imaging and retire my current one for visual with my lad. I've got a Coma Corrector 3 which seems to cover my relatively small sensor well enough, but I reckon if I do get the extender, then I'd probably stick with that most of the time because if I want wide-field I have an Askar FRA400. 

Is "downsampling in post" the same as processing as Vlaiv recommends?

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2 hours ago, osbourne one-nil said:

Nice! I don't quite get Vixen's relatively low-key profile. Perhaps they don't cost enough? Make them Takahashi prices and you're on to a winner!

I think my scope is about 8 years old but I like it so much I am almost tempted to buy a brand new one for imaging and retire my current one for visual with my lad. I've got a Coma Corrector 3 which seems to cover my relatively small sensor well enough, but I reckon if I do get the extender, then I'd probably stick with that most of the time because if I want wide-field I have an Askar FRA400. 

Is "downsampling in post" the same as processing as Vlaiv recommends?

The weight was a big plus point for me. I only have an HEQ5-Pro mount, and i need to set up each time, so the fact it only weighs 5.3Kg was the clincher for me. My one really is in need of some TLC. though. The mirror needs removed and given a good clean, and the primary collimation screws are starting to rust (although they still work, for now) so one of the most important jobs will be to find suitable replacement ones for those (and maybe even the secondary ones too). I'm hoping the screws are the same size as other more popular scopes, that way i might be able to get some Bobs Knobs. 

If i had an OSC camera, i would do things a little differently to Vlaiv's approach, although how much of a difference it would make i don't know. I would stack using 'Bayer Drizzle', as you are dithering and will have lots of subs due to the short sub length. Bayer Drizzle also doesn't do any interpolation. Then you can just resize the stack by 50% or do it in post (i.e when processing). If you have something like APP or P.I then you can re-sample using a good algorithm such as lancsoz-3 but if not then even just a bog standard re-size in PS would get the job done ok. 

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Ah -  someone else has suggested drizzling to me so I shall watch a few Youtube videos on it tonight. Thanks!

I've got mine set up with a big old Losmandy plate on the back which carries my 50mm guide scope and the ASIair Pro, but even with it all rigged out, it's still weighing in at less than 10kg. I'm tempted to go down the OSG route to keep the weight down but that's more for my benefit than the mount's.. The SXD2 is rated for 15kg but I think that's a very conservative minimum, knowing Japanese manufacturers. 

IMG_3193 (1).JPG

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On 15/02/2022 at 16:37, osbourne one-nil said:

Ah -  someone else has suggested drizzling to me so I shall watch a few Youtube videos on it tonight. Thanks!

I've got mine set up with a big old Losmandy plate on the back which carries my 50mm guide scope and the ASIair Pro, but even with it all rigged out, it's still weighing in at less than 10kg. I'm tempted to go down the OSG route to keep the weight down but that's more for my benefit than the mount's.. The SXD2 is rated for 15kg but I think that's a very conservative minimum, knowing Japanese manufacturers. 

IMG_3193 (1).JPG

Very nice. Yours is certainly a lot shinier than mine i must say! lol. 

I can see a red thumb screw at the Primary end. Did yours come with these or did you replace them? 

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The previous owner installed them it seems - good old Bob's Knobs. There's a rumour that these things never need collimating but that's not true! They don't need much tweaking but they do need some...unless putting the Bob's Knobs on makes it more prone to needing tweaks?

 

 

IMG_3812.JPG

IMG_3813.JPG

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  • 2 weeks later...

Here's an image I got last night with my R200SS.. It's 54 x 60" exposures and I think collimation was ok. Would an image taken through a refractor be any "better" or just different? Of course, a refractor at 800mm focal length would be slower, but I wonder if the image would be vastly different?

Don't judge the processing too much...I have mancovid and I did it very quickly. 

 

 

Leo Triplet PIS copy.jpg

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